1. Field of the Invention
The present invention relates to processing of digital image data, and more particularly to edge processing for character images.
2. Description of Prior Art
An image processor processes digital image data obtained by reading a document for printing. An image of the document is reproduced based upon the digital print data.
The image processor carries out various processes on the digital image data obtained in order to reproduce the image better. For reproduction of a character image in a character document, it is preferable to carry out edge emphasis on edge portions of characters. Therefore, various edge judgment methods and data emphasis methods based on the results of the edge judgment have been proposed.
In the case when a character image includes an edge oriented in a diagonal direction, after such an edge has been subjected to edge emphasis, the resulting image has jaggies. Consequently, the reproduced image is degraded on the image quality as compared with the original image. This is because the image is read and reproduced in the unit of pixel. It is desirable to reproduce the edge portions of characters more smoothly.
When an image is formed, it is proposed to divide a pixel into a plurality of sub-pixels in the main scan direction (for example, Japanese Patent laid-open Publication No. 9-240053/1997). This improves the resolution of the image, and fine expression of the image can be realized. In this case, smoothing is carried out on character edge portions by taking into account the gradation of adjacent pixels with respect to the gradation of a target pixel. Even in the case of a bi-level image, an image of multi-level gradation can be formed. Moreover, the center of gravity in density can be changed within the pixel by changing the density for each sub-pixel. However, the application of the smoothing on character edge portions raises a problem that fine lines in a multi-level gradation image have narrower line widths after the smoothing.
Another problem on the division of a pixel into a plurality of sub-pixels is that when input gradation data is bi-level data, the position of the center of gravity in density is always fixed. Therefore, even if the density is controlled in the unit of sub-pixel, it is not possible to obtain an advantage of smoothed edges.